The present invention relates to the field of semiconductor lasers.
Silicon lasers do not exist at present, and are the "missing link" in Si photonics technology. New and practical Si laser designs are needed which, when implemented, will "complete" the Si photonics technology. Designs are especially wanted that "stay within Group IV"; that is, structures wherein 100% of the laser materials are made from some combination of the elemental semiconductors Si, Ge, C, Sn, structures whose fabrication is compatible with known Group IV epitaxy techniques and with monolithic integration on advanced silicon electronic integrated circuits.
Although workers skilled in silicon photonics have made vague suggestions that the quantum-well (QW) and superlattice (SL) approach is viable for silicon-based lasing, published specifics as to how this would be done are lacking.
Several patents exist on III-V semiconductor quantum-cascade lasers such as U.S. Pat. Nos. 5,570,386 and 5,509,025. However, it is not obvious to skilled workers how to construct a Group IV quantum cascade laser. The reason is that the phonon scattering properties and other relevant physical properties differ considerably between III-V and Group IV materials.
U.S. Pat. No. 5,442,205 (Brasen), U.S. Pat. No. 5,221,413 (Brasen), and U.S. Pat. No. 4,959,694 (Gell) have taught strained heterostructures consisting of pure Ge layers and pure Si layers grown upon a compositionally graded buffer layer of Si.sub.1-x Ge.sub.x on a silicon substrate. The first two patents have taught this strained-layer system as a foundation for transistors and light emitting diodes, but not lasers. Thus, symmetrically strained Ge--Si/SiGe/Si "in-and-of itself" or as a foundation structure is known in the art. Rather, the focus of the present invention involves specific superlattice (SL) laser embodiments having p-SiGe tunnel injector-emitters, p-SiGe collectors and appropriate electrical pumping biases. The Gell patent teaches a strict relationship between the thickness of the Ge and Si layers and a specific Si.sub.1-x Ge.sub.x buffer with &gt;60% Ge, in order to produce a superlattice that has special "zone folding" properties which induce a direct bandgap between the valence and conduction minibands, whereas the bulk materials possess an indirect bandgap unsuitable for lasing. While the specialized superlattice of Gell might be employed for a band to band laser, his patent does not mention applications of optical transitions between valence minibands or between conduction minibands. Yet, those intersubband transitions are the exclusive focus of this patent and not band-to-band effects. In addition, we focus on Si.sub.1-x Ge.sub.x buffers with less than 60% Ge, not more than 60%. The present invention also differs from Gell's teachings because we do not require his complex numeric relation between Ge and Si monolayers.
An invention disclosure of T. K. Gaylord et al., "Silicon-based optical emitters, detectors, modulators, and switches using bound and quasi-bound energy levels," Georgia Tech Record of Invention No. 1710 of Jun. 1, 1996, described in JSEP Annual Report for DAA H04-96-1-0161, describes a light emitter having an active gain region that employs Si.sub.1-x Ge.sub.x alloys for quantum wells and Si.sub.1-y Ge.sub.y alloys for barriers. In contrast, the present invention, with one exception, does not employ any alloys within the active gain region. The Georgia Tech invention disclosure teaches asymmetric strain in the active MQW, an asymmetry that we avoid. The Georgia Tech invention uses quasibound or above-barrier states, whereas the present invention uses bound states only, and thus also differs from U.S. Pat. No. 5,386,126 to G. N. Henderson et al., entitled "Semiconductor devices based on optical transitions between quasibound energy levels."
Workers in silicon photonics have written many papers on the Si-based asymmetrically strained system of Si.sub.1-x Ge.sub.x quantum wells with unstrained Si barriers. The vague implication in these publications is that asymmetric SiGe/Si multiple-quantum wells (MQWS) could be useful for lasing. However, there is a basic problem with such systems that mitigates against their use in lasers: the height of a MQW stack of asymmetrically strained SiGe/Si is limited to a small height, about 30 nm due to the build-up of net strain in the MQW. The present invention solves this problem with symmetric strain in each period and net-zero strain in the MQW stack. This removes the height limit and allows stacks of 500 to 1000 nm.
Another problem with SiGe/Si is the &lt;400 meV valence band offset, which restricts the intersubband optical transition wavelength to wavelengths longer than about 6 .mu.m. In contrast, relatively short near-infrared wavelengths are allowed in the present Ge--Si invention due to the large 770 meV valence band offset in symmetric Ge--Si permitting intersubband lasing at wavelengths as short as 3 .mu.m.
Another problem is the perceived need for coupled quantum wells in the active region of an intersubband laser. Most of the proposals floated for intersubband lasers (see the review in SPIE Proceedings, volume 2397) require coupled wells: a complex and difficult scheme to execute in practice. The present invention solves this problem by using a simple individual square well in each active period. The one-well structures are made possible by our local-in-k-space population inversion approach discussed below.
Another problem is that people do not know how to build a far-infrared laser (wavelengths of 30 to 100 .mu.m or more) in any intersubband material, let alone in Group IV material. The present invention solves this problem by providing a long-wave Ge--Si staircase laser.
Another problem is: how can local-in-k population inversion be obtained in Si or Ge quantum wells? That problem is solved in accordance with the present invention with the aid of our detailed quantum-mechanical calculations of in-plane dispersion for Ge subbands.